In recent years, frequent disasters and climate change that are probably caused by global warming have had a tremendous influence on agricultural production, living conditions, energy consumption, etc. This global warming is considered to be caused by an increase in the presence of greenhouse gases such as carbon dioxide, methane, nitrous oxide, and CFCs in the atmosphere, which is associated with an increase in human activity. The main gas among the greenhouse gases is atmospheric carbon dioxide. To prevent global warming, the Global Warming Prevention Conference (COP3) was held in Kyoto in December, 1997. The Kyoto Protocol, which was adopted at the Global Warming Prevention Conference, went into effect on Feb. 16, 2005. There is thus an urgent need for a measure to reduce carbon dioxide emissions.
Examples of sources of carbon dioxide include thermal power plants, boilers of factories or kilns of cement plants using coal, heavy oil, natural gas, or the like as a fuel; blast furnaces of iron mills where iron oxide is reduced using coke; and transportation equipment using gasoline, heavy oil, light oil or the like as a fuel, such as automobiles, ships, and aircrafts. Except for transportation equipment, these sources of carbon dioxide are fixed facilities, which are expected to be easily adapted to implement CO2 emission reduction measures.
Several methods for recovering carbon dioxide from gas are already known, and a wide variety of methods are currently in development.
For example, one method that has been in development since the 1930s comprises bringing a gas containing carbon dioxide into contact with an aqueous alkanolamine solution in an absorption tower to absorb carbon dioxide into the solution, and then heating the aqueous solution for CO2 recovery to separate and recover carbon dioxide using a regenerator. This method is now used in urea synthesis plant towers. This method is economical, and can be carried out on a large scale.
Examples of alkanolamines include monoethanolamine (hereinafter sometimes referred to as MEA), diethanolamine (hereinafter sometimes referred to as DEA), triethanolamine (hereinafter sometimes referred to as TEA), N-methyldiethanolamine (hereinafter sometimes referred to as MDEA), diisopropanolamine (DIPA), diethylene glycol amine (DGA), and the like. MEA is typically used.
However, the use of such an aqueous alkanolamine solution as an absorbing solution requires either the use of an expensive anti-corrosion steel as the equipment material or the reduction of the amine concentration in the absorbing solution, because the solution is highly corrosive to the equipment material. Furthermore, because it is difficult to separate the absorbed carbon dioxide, the solution must be heated at a high separation temperature of 120° C. to separate and recover carbon dioxide. Another disadvantage is the high energy, i.e., 20 kcal/mol CO2, required to separate carbon dioxide from the absorbing solution. For example, to recover carbon dioxide in an electric power plant by this method, extra energy that corresponds to 20% of the generated electric power is required. In an age when the reduction of carbon dioxide emissions, energy saving, and resource saving are desired, this high energy consumption is a major impediment to the realization of a carbon dioxide absorption and recovery system.
For example, Patent Document 1 describes a method of removing carbon dioxide from combustion exhaust gas, comprising bringing combustion exhaust gas at atmospheric pressure into contact with an aqueous solution of a so-called “hindered amine” to absorb carbon dioxide into the aqueous solution. The “hindered amine” has a steric hindrance around the amino group, such as in an alkyl group.
Patent Document 1 describes using 2-methylaminoethanol (hereinafter sometimes referred to as MAE) or 2-ethylaminoethanol (hereinafter sometimes referred to as EAE) as a hindered amine in the Examples, and further states that an aqueous solution of MAE or EAE is preferably used to absorb carbon dioxide. Although not described in the Examples, other amines such as 2-isopropylaminoethanol (hereinafter sometimes referred to as “IPAE”) are also mentioned as examples of hindered amines in Patent Document 1.
Patent Document 2 describes a method of recovering carbon dioxide, comprising the step of bringing a mixed gas into contact with an aqueous amine solution to absorb carbon dioxide into the aqueous solution, and the step of separating carbon dioxide from the aqueous solution.
Patent Document 2 describes, as aqueous amine solutions, compounds containing a secondary amino group attached to a secondary or tertiary carbon, or compounds containing a primary amine attached to a tertiary carbon, such as 2-methylpiperazine (hereinafter sometimes referred to as 2MPZ) and 2-amino-2-methyl-1-propanol (hereinafter sometimes referred to as AMP).
Patent Document 3 describes a method for removing carbon dioxide from combustion exhaust gas, which comprises the step of bringing the combustion exhaust gas into contact with a mixed aqueous solution containing 100 parts by weight of a first amine compound selected from the following compounds (A) and (C) shown below, and 1 to 25 parts by weight of a second amine compound selected from compounds (D) to (I) shown below: (A) a compound containing one alcoholic hydroxyl group and a primary amino group in the molecule, the primary amino group being bonded to a tertiary carbon atom having two unsubstituted alkyl groups; (C) diethanolamine; (D) piperazine, (E) piperidine; (F) morpholine; (G) glycine; (H) 2-piperidinoethanol; and (I) a compound that has one alcoholic hydroxyl group and a secondary amino group in the molecule, the second amino group having an unsubstituted alkyl group having 3 or less carbon atoms, and a nitrogen atom bonded to a carbon-chain group containing two or more carbon atoms inclusive of the bonded carbon atom. Patent Document 3 describes ethylaminoethanol and 2-methylaminoethanol as preferable examples of amino compounds denoted by (I).
Patent Document 4 describes a method of removing carbon dioxide from combustion exhaust gas, comprising bringing combustion exhaust gas at atmospheric pressure into contact with an aqueous amine mixed solution containing a secondary amine and a tertiary amine each having a concentration of 10 to 45 wt. %. In the Reference Examples of Patent Document 4, 30 wt. % aqueous solutions of 2-isopropylaminoethanol or others are described as experimental examples.
The method of recovering carbon dioxide comprises removal of carbon dioxide from combustion exhaust gas, i.e., a step of absorbing carbon dioxide into an aqueous solution, and a step of separating carbon dioxide from the aqueous solution. Accordingly, to efficiently recover carbon dioxide, the separation step as well as the absorption step must be performed with high efficiency.
Although many attempts have been made to increase the efficiency of the step for absorbing carbon dioxide into an aqueous solution as described above, either carbon dioxide separation efficiency was not considered, or the only available methods were insufficient in terms of the amount of CO2 separated or the separation rate. Therefore, the balance between CO2 absorption and CO2 separation was poor in known methods for recovering carbon dioxide, which resulted in low carbon dioxide recovery efficiency.
Another important issue is reduction of the heat of the carbon dioxide absorption reaction, i.e., the heat used to separate carbon dioxide, in order to attain CO2 recovery at low cost.
Patent Document 1: Japanese Patent No. 2,871,334
Patent Document 2: U.S. Pat. No. 4,112,052
Patent Document 3: Japanese Patent No. 2,871,335
Patent Document 4: Japanese Patent No. 3,197,183